Alpha 1-Adrenergic receptors of a nonfusing muscle cell line (BC3H1) have been identified on intact and broken-cell preparations by using the high-affinity antagonist [3H]prazosin. In intact cells, both equilibrium and kinetic studies gave KD values in the range of 0.07-0.12 nM. The maximal number of binding sites determined by Scatchard analysis was about 85,000 +/- 9,000 sites/cell. Antagonists bound to the [3H]prazosin binding sites with Michaelis-Menten characteristics, and their specificity was typical of alpha 1-adrenergic receptors. No significant modification of antagonist binding was observed either after cell disruption or by lowering incubation temperature to 4 degrees. In contrast, in intact cells at 37 degrees, agonist competition curves were shallow with Hill coefficients of less than 1. The heterogeneity of [3H]prazosin binding sites toward (--)-norepinephrine also appeared in [3H]prazosin saturation experiments carried out in the absence and in the presence of this agonist. After cell disruption, the EC50 values of agonist competition curves decreased and Hill coefficients were close to 1. When the temperature was lowered from 37 degrees to 4 degrees, the affinity for (--)-norepinephrine in intact cells increased dramatically by 10,000 times and the Hill coefficient of the competition curve was equal to unity. This affinity shift induced by temperature was not so important in broken-cell preparations (50 times).